Free and clear laundry soap. Any brand will work. Make sure there are no scents or dyes and put them in the dryer without any additive as well or hang dry.
I'm new here and not an expert on any of this, but I've read elsewhere on this site, just as has been suggested here already, that liquid laundry detergent in the washer works fine. I have been washing my recently purchased MF towels that way over the past week or so without a problem. The only caveat I have seen mentioned is to avoid the use of fabric softeners which I have done.
fill tub with water and liquid laundry detergent and i ad 1 cup of bleach then mix it up real good then ad mf foam and wool pads all at the same time been doing this for last 4 yrs. ps hang dry only and pads out side in the sun
but i still use micro restore and on occasion, for stains, shout.
oh i forgot to add that i use nothing more than dawn to clean my pads! works so well but then again i only use OPT products which are notoriously easy to clean out of pads. i don't even have any trouble with poli-seal because i use very little per application. a little goes a LONG way with poli-seal.